FIG. 3 illustrates a conventional overheat detecting circuit of the foregoing type. As illustrated therein, a MOSFET transistor 31 and a MOSFET transistor 32 constitute a current mirror, a gate terminal and a drain terminal of the MOSFET 31 are each connected to a negative power source V.sub.SS 38 through a resistance 33.
The drain terminal of the MOSFET 32 is connected to negative power source VSS38 through a six-stage diode 34, and the dropping voltage 6VF corresponding to the six-stage diode (VF is a forward voltage of a diode) and the output VR of a band gap voltage source circuit (hereinafter referred to as "BGR circuit", and BGR stand for Band Gap Reference) 35 which is connected between a positive power source V.sub.DD and the negative power source V.sub.SS are each connected to the input terminal of a comparator 36.
As will be described later, since the output VR of the BGR circuit 35 is generated by synthesizing a V.sub.BE -dependent type voltage source that is the temperature coefficient of which is negative and a thermal voltage(=kT/q)-dependent type power source voltage that is the temperature coefficient of which is positive, and can be arranged so that it has no temperature coefficient, the output VR becomes constant independently from the fluctuation of the power source voltage and the temperature variation.
A constant current I.sub.S which flows through a resistance R.sub.33 is given with the resistance value of a resistance 33 taken as R.sub.33 and with the MOSFETs 31, 32 forming the current mirror having the same characteristics. EQU I.sub.S =[(V.sub.DD -V.sub.SS)-V.sub.T ]/R.sub.33 formula ( 1)
where: V.sub.T denotes an invertion voltage or threshold voltage of the MOSFET 31.
The relationship between the dropping voltage 6VF and the output voltage VR of the BGR circuit 35 as this constant current I.sub.S flows through the diode 34 is set so as to satisfy the following condition at the room temperature. EQU VR&lt;6VF formula (2)
Since the dropping voltage 6VF has a negative coefficient as the current is made to flow through the diode 34, if the ambient temperature is elevated, then the value of the dropping voltage 6VF is lowered. Therefore, if the temperature rises above a predetermined ambient temperature, then the dropping voltage 6VF becomes less than the output VR of the BGR circuit 35, and an output of a comparator 36 is inverted. Based on the change of the output signal of the comparator 36, the overheated condition of the circuit is detected. In order to protect the integrated circuit, the cut-off control over the circuit operation and the like is carried out by means of a control circuit and the like (not shown).
As the BGR circuit 35, a circuit shown, for example, in FIG. 4, is used. In FIG. 4, at the stably operating point, the differential voltage of the differential amplifier 41 between the inverted input and the non-inverted input becomes zero, with the dropping voltage of the resistances R.sub.1 and R.sub.2 equaling each other, the currents I.sub.1 and I.sub.2 satisfying the relationship: I.sub.1 .times.R.sub.1 =I.sub.2 .times.R.sub.2 and the node voltages V.sub.A and V.sub.B equaling each other. Incidentally, in place of the diodes 42, 43 (shown), transistors having the collector and the base connected may be arranged.
If the saturation current of the diode is set to I.sub.S, a forward current V.sub.BE of the diode is given with (kT/q)Vn (I/Is) from its rectifying characteristic, and the dropping voltage V.sub.R2 of the resistor R.sub.2 is given according to the following formula (3). ##EQU1##
where: k: Boltzman's constant, T: absolute temperature, q: amount of electrical charges of electrons.
As illustrated in FIG. 4, the output voltage V.sub.out of the BGR circuit becomes a sum of the forward voltage 2V.sub.BE of the two stage-diode,and the dropping voltage V.sub.R1 (=I.sub.1 .times.R.sub.1) of the resistance R.sub.1, and is provided according to the following formula (4). That is, the output voltage V.sub.out is given as the synthesis of the V.sub.BE -dependent type voltage source having the negative temperature coefficient and the thermal voltage-dependent type voltage source having the positive temperature coefficient. ##EQU2##
When the temperature coefficient of the output voltage V.sub.out is set to zero, the output voltage is given according to the following formula (5), and the output of the BGR circuit can be set substantially constant independently of the power source voltage and the temperature. EQU V.sub.out =2V.sub.GO +2V.sub.TO .times.(.gamma.-.alpha.) formula (5)
Here, V.sub.GO denotes a band gap voltage of silicon, and equals 1.205 V, and V.sub.TO (=kT.sub.0 /q) denotes the thermal voltage at the temperature of T.sub.0 (26 mV at the room temperature) and .alpha., .gamma. each denotes a predetermined constant.
The foregoing conventional overheat detecting circuit has a disadvantage that the current value I.sub.S of the constant current suffers a variation due to the fluctuating power source voltage and that the detected temperature also fluctuates.
That is, in the foregoing conventional overheat detecting circuit, since the fluctuation of the power source voltage causes the value of (V.sub.DD -V.sub.SS) in the foregoing formula (1) to change, the temperature at which the overheating is detected fluctuates, and a precise overheat detecting is impossible.
Further, in the foregoing conventional overheat detecting circuit, there was a problem of the fluctuation of the detected temperature which is caused by the fluctuation of the element characteristic as occurs when the semiconductor integrated circuit is manufactured and, in particular, the fluctuation and the dispersion of the output voltage VR of the BGR circuit 35 cannot be properly suppressed.
Prior art overheat detecting circuits are described in Japanese Patent Laid-Open Publication Nos. Sho 64-15623, Hei 4-30609. But these prior art publications do not describe the BGR circuit and the like.